1. Field of the Invention
The present invention relates to a downconverter for performing frequency conversion in a receiver and an upconverter for performing frequency conversion in a transmitter.
2. Description of the Related Art
(Background Technology of Downconverter of Low-Intermediate Frequency (IF) Scheme)
A radio communicator which functions both as a receiver and a transmitter (like a mobile phone), receives a Radio Frequency (RF) signal carrying speech content and data communication content and converts the received RF signal to a frequency to be input to a demodulator of a receiver, i.e., a downconverter. There are three front-end schemes for selecting a channel to select a target signal in the receiver including a heterodyne scheme for converting an RF signal to an Intermediate Frequency (IF) signal, a zero-IF scheme (referred to as a direct conversion or homodyne scheme) for converting an RF to Direct Current (DC), and a low-IF scheme for converting an RF signal to an IF signal slightly away from DC using an image rejection mixer (or a half-complex mixer for a real input and a complex output) for rejecting an image frequency signal.
The background in which the above-described three schemes are present will be briefly described. The simplest scheme for frequency-converting an RF signal to an original signal is the zero-IF scheme for performing direct frequency conversion to a baseband signal by multiplying the frequency of the RF signal by the same frequency.
However, an expensive Phase Locked Loop (PLL) circuit with very high precision is required to ensure signal orthogonality and other conditions such that the zero-IF scheme is realized in a high frequency domain of an RF signal. Accordingly, a frequency conversion scheme has been proposed which requires a lower component precision than the zero-IF scheme. The proposed scheme performs the frequency conversion by lowering a frequency step by step. This frequency conversion scheme includes the heterodyne scheme and the low-IF scheme.
The heterodyne scheme increases the frequency of an IF signal and increases a difference between a frequency of a target signal and an image frequency in an RF signal processor before frequency conversion, thereby rejecting an image frequency signal by means of an RF filter and avoiding interference of the image frequency signal. Here, the image frequency is present in a low frequency side separated by twice the frequency of the IF signal from the target signal frequency, and influences the target signal at the time of down-conversion.
As a concrete example of the heterodyne scheme, a full-duplex radio device for simultaneously performing transmission and reception operations rejects a transmission frequency signal or a transmission signal (hereinafter, an image frequency signal) close to an image frequency when a local signal is common between transmission and reception. If a filter of an RF signal (hereinafter, referred to as an RF filter) cannot completely reject a generated image frequency signal when the RF signal is converted to an IF signal, a frequency of the IF signal is changed between all radio communication schemes and a frequency of the image frequency signal is changed, such that the RF filter can reject the image frequency signal. For this reason, a multi-mode radio device for supporting multiple communication schemes changes the frequency of the IF signal in every mode according to channel bandwidths different between the modes (or communication schemes). Moreover, the multi-mode radio device needs to prepare a filter of the IF signal (hereinafter, referred to as an IF filter) different between center frequencies or pass frequencies of the modes. In this case, there is a problem in that circuit size significantly increases.
On the other hand, the low-IF scheme performs frequency conversion using an image rejection mixer 810 (corresponding to a mixer for a real input and a complex output) configured by a mixer-I 814 and a mixer-Q 815 that are provided with a multiplier connected to a local oscillator 813 for outputting a local signal in a downconverter 9 of the low-IF scheme as illustrated in FIG. 21, respectively. The local oscillator 813 and the image rejection mixer 810 configure a frequency converter. An undesired signal present in a symmetric position of the low frequency side, corresponding to a frequency value of the IF signal with respect to the frequency of the target signal (i.e., an image frequency signal), is rejected on the basis of a frequency of the local signal without depending on frequency characteristics of the RF and IF filters. Here, a rejection ratio of an image frequency signal is expressed by an image rejection ratio to be described below. The image rejection ratio can decrease the frequency of the IF signal because dependency on the characteristics of the RF filter is low.
When the frequency of the IF signal is equal to a channel interval, an image frequency of a target channel is the next channel neighboring to the target channel.
For example, the downconverter 9 satisfies the specification of an associated radio scheme when an image rejection ratio associated with a requirement specification such as blocking for an image frequency signal separated by twice a frequency of an associated IF signal from a frequency of a target IF signal is less than the image rejection ratio of the downconverter 9 of an associated low-IF scheme in a radio communication scheme using the downconverter.
Because the low-IF scheme can decrease the frequency of the IF signal, the IF filter can be configured by an active filter and an integrated circuit (IC) device can be easily miniaturized. Because the frequency of the IF signal does not need to be changed according to each radio communication scheme in the multi-mode radio device, the IF filter can be commonly employed.
If the image frequency signal is present in the symmetric position of the low frequency side corresponding to the frequency value of the IF signal with respect to the frequency of the target signal on the basis of the frequency of the local signal as described above, it means that the frequency corresponding to twice the frequency of the IF signal becomes a frequency interval between the frequency of the target signal and the image frequency.
Because the channel bandwidths are different between the communication schemes in the above-described multi-mode radio device, the bandwidth of the IF filter must be changed according to each radio communication scheme. However, the low-IF scheme can easily vary characteristics of the IF filter using a transconductance-capacitor (gmC) filter for varying transconductance (gm) of a transistor, if needed. When the low-IF scheme is applied to the multi-mode radio device, one IF filter can be provided because multiple IF filters are not needed as in the heterodyne scheme, such that the multi-mode radio device can be realized in a small circuit size.
Even though the low-IF scheme outperforms the heterodyne scheme as described above, it may ensure only the image rejection ratio of about 30 dB as described in Philips, SA1920 datasheet and Philips, SA1921 datasheet. The image rejection ratio of about 30 dB is not a value capable of being easily realized. To realize the image rejection ratio of about 30 dB, a size of an associated transistor needs to be increased such that the image rejection ratio of a mixer due to performance variation of a used transistor can be prevented from being reduced. Increasing the size of the transition causes all characteristics, except the image rejection ratio, being degraded due to an increase in power consumption and a decrease in a transition frequency, fT.
The low-IF scheme can be applied to the radio communication scheme whose specifications such as blocking for an image frequency signal are rigid even when the image rejection ratio of about 30 dB can be realized. However, there is a problem in that an associated requirement specification cannot be satisfied and the low-IF scheme cannot be applied, when a robustness to interference ratio of more than 30 dB is required.
However, the low-IF scheme can be applied because a requirement specification for interference robustness such as blocking for an image frequency signal at a frequency within 300 kHz from a target signal frequency is only 18 dB in Global System for Mobile Communication (GSM).
On the other hand, because a requirement specification for interference robustness for an adjacent channel separated by 5 MHz from a frequency of a target signal is 33 dB in Wideband Code Division Multiple Access (W-CDMA), the low-If scheme provides borderline performance with respect to the image rejection ratio of 30 dB as described above when practical use is considered. A need exists for precision improvement for improving selection of a mixer used in a device or an image rejection ratio such that the low-IF scheme can satisfy an associated requirement specification. To achieve the precision improvement, a large chip area may be required and costs may increase.
The GSM or W-CDMA uses a digital tuner or a software radio front-end for converting a frequency in an RF part and selecting a channel from a plurality of channels in a digital part. In this case, a requirement specification of interference robustness such as blocking for an image frequency signal at a frequency separated by more than 300 kHz from a frequency of a target signal is more than 50 dB, for example, in the GSM. When the same requirement specification exceeds the image rejection ratio capable of being realized by the image rejection mixer also in the W-CDMA, the channel selection of the digital part is actually impossible. Accordingly, the low-IF scheme cannot be applied to the digital tuner or the software radio front-end.
To address these problems in the low-IF scheme, four means can be considered for obtaining the image rejection ratio of more than 40 dB using the above-described image rejection mixer.
The first means rejects an image frequency signal with an RF filter by increasing a frequency of an IF signal and increasing a difference between a target signal frequency and an image frequency in the RF part before frequency conversion.
The second means corrects characteristics of the image rejection mixer through a correction process based on a digital process described in Japanese Patent Laid-Open No. 2002-246847 and Japanese Patent Laid-Open No. Hei 6-188928 [PLEASE CONFIRM PATENT NUMBER], and a correction process based on an analog circuit process described in Japanese Patent No. 2988277 and Japanese Patent Laid-Open No. 2000-224497.
The third means rejects an image frequency signal by providing a phase shifter in an RF part, obtaining a phase difference of 90 degrees in an associated phase shifter, generating a complex RF signal, and performing frequency conversion by complex multiplication of the complex RF signal by a complex local signal as described in “Mixer Topology Selection for a Multi-Standard High Image-Reject Front-End”, Vojkan Vidojkovic, Johan van der Tang, Arjan Leeuwen burgh and Arthur van Roermumd, ProRISC Workshop on Circuits, Systems and Signal Processing, pp. 526-530, 2002 (hereinafter “Mixer Topology Selection for a Multi-Standard High End” and FIG. 3.25(b) of “CMOS WIRELESS TRANSCEIVER DESIGN”, Jan Crols, Michiel Steyaert, Kluwer, International Series in Engineering and Computer Science, 1997 and FIG. 3.25(b) (hereinafter “CMOS WIRELESS TRANSCEIVER DESIGN”).
The fourth means rejects an image frequency signal by frequency-converting an RF signal, generating a complex signal, and performing complex multiplication with a complex local signal through a mixer using the complex local signal as illustrated in FIG. 3.28 and FIG. 3.31 of “CMOS WIRELESS TRANSCEIVER DESIGN”.
However, the four means include the following problems with the downconverter. The first means has a problem in that power consumption increases due to a clock increase in an analog-to-digital converter (ADC) for converting an IF signal to a digital signal and a digital signal processor for processing an output of an associated ADC. This is the first problem of the downconverter. To address this problem, a sub-nyquist sampling technique is used for the clock reduction in the ADC as is well known. In this case, an input frequency band of the ADC is widened, such that power consumption increases. Accordingly, it is difficult for a sub-nyquist sampling technique to be actually adopted.
The second means has a problem in that power consumption increases according to a computational process in a digital part for a correction based on a digital process. The second means has another problem in that a size of a correction circuit, for a correction based on an analog process, increases and correction precision is bad.
The third means has a problem in that loss occurs in the phase shifter. The loss in the phase shifter increases, for example, when a degree of the phase shifter is increased to widen a band. Due to this loss, reception sensitivity is degraded. The third means has another problem in that practical precision cannot be obtained in the phase shifter configured by a Resistor-Capacitor (RC) circuit when input/output impedance is considered because R and C values are small in the RF of a high frequency.
The fourth means has problems in that power consumption increases because the number of mixers and the number of local signal oscillators are increased to generate complex signals from the mixers using complex local signals and spurious reception occurs due to the increased number of local signal oscillators.
In addition to the above-described first to fourth problems, the low-IF scheme has a problem in that a circuit size increases because a local oscillator 813 for outputting orthogonal local signals of sin and cos and two multipliers (i.e., mixers 814 and 815) are needed, as illustrated in FIG. 21. The heterodyne scheme requiring only a single local signal and a single mixer has a problem in that power consumption increases.
A. Background Technology of Upconverter of Low-IF Scheme
A transmitter of a radio communicator such as a mobile phone or so on generates a real IF signal by mixing a complex baseband signal with a complex local signal and generates a real RF signal by mixing the real IF signal with a real local signal according to an upconverter function, such that the baseband signal including speech content and data communication content is converted to the RF signal.
To reject an image frequency signal of an IF signal in an RF filter of an upconverter, an IF signal frequency needs to be increased according to a broad system bandwidth and needs to be further increased according to a broad RF band corresponding to a broad channel band due to a high communication rate. Therefore, there is a problem in that cost and power consumption increase in an IF signal processor. On the contrary, there is a problem in that a rigid requirement specification is applied when an IF signal frequency is reduced.
To address these problems, the upconverter can have a structure (hereinafter, referred to as a first structure) for rejecting an image frequency signal while reducing an IF signal frequency by converting a complex baseband signal to a complex IF signal in a full-complex mixer and mixing an associated complex IF signal with a complex local signal in an image rejection mixer (hereinafter, referred to as a half-complex mixer). According to the first structure, a requirement specification for an attenuation amount of a stop band of the RF filter is mitigated. The first structure requires only a one-step SAW filter rather than two-step SAW filters conventionally needed for an RF signal.
From Philips, SA1920 datasheet and Philips, SA1921 datasheet, it can be seen that an image frequency signal of −30 dBc is estimated as a spurious transmission component in terms of the performance of an image rejection ratio of the full-complex mixer available for reception in the receiver. In this performance, the spurious transmission component may exceed the specification of an allowable mask (hereinafter, referred to as a spurious mask). To satisfy the specification, an RF filter is needed to reject an image frequency signal. When the RF filter is required, a relation between the requirement specification for the RF filter and the IF signal frequency needs to be considered. As a result, there is a problem in that a high IF signal frequency occurs because the first structure cannot reduce the IF signal frequency.
Therefore, a structure (hereinafter, referred to as a second structure) for the low-IF scheme used in the above-described downconverter can be considered as special means for addressing the problem of the high IF signal frequency. FIG. 32 illustrates a conventional upconverter using the low-IF scheme. When the second structure is taken, an RF filter for rejecting an image frequency signal of an IF signal is unnecessary due to the effect of rejecting the image frequency signal in the half-complex mixer 400. Alternatively, a SAW filter of an RF signal may not be required according to conditions of a requirement specification when the requirement specification for the SAW filter of the RF signal is significantly mitigated.
However, the image frequency signal appears at a target frequency because the upconverter of the second structure cannot completely remove the image frequency signal. FIG. 33 illustrates a spectrum of a complex IF signal (S35 of FIG. 33) with a center frequency of 5 MHz frequency, converted from a Double Side Band (DSB) signal with a carrier interval of 1.6 MHz of a complex baseband in an upconverter 35 of the second structure of FIG. 32. FIG. 34 illustrates a spectrum of a real signal output when the complex IF signal illustrated in FIG. 33 is mixed with a complex local signal (of 795 MHz) in which an error of 10% is present between amplitudes of a real axis signal I, corresponding to a real part (of an in-phase component), and an imaginary axis signal Q, corresponding to an imaginary part (of a quadrature phase component). In FIG. 34, an image frequency signal of −26 dBc (S37 in FIG. 34) occurs with respect to a target signal (800 MHz) S36 at the image frequency (790 MHz).
If only the image rejection ratio of about −30 dBc can be ensured, a spurious mask near a target signal exceeds an associated specification in the upconverter of the second structure as in a mixer shown in the above-described Phillips, SA1920 datasheet and Phillips, SA1921 datasheet. For example, there is a problem in that an associated specification may not be stably satisfied because the image rejection ratio may be reduced due to performance variation of the half-complex mixer or variation of environment conditions, even though the specification of an associated spurious mask can be almost satisfied.
To reduce degradation of the image rejection ratio, it may be attempted to limit the variation in manufacturing error of a transistor used for a mixer that results from increasing a transistor size. When the transistor size increases, the power consumption of the transistor increases, the transition frequency, fT, decreases, and all characteristics except the image rejection ratio are degraded. Because of the inaccuracy of an analog circuit, it is difficult for an image rejection ratio to be satisfied and therefore it is difficult for the method for increasing the transistor size to be taken.
To address a problem associated with the upconverters of the above-described first and second structures, a third structure can be considered for a transmitter of the low-IF scheme according to a signal process using a polyphase filter used in a receiver illustrated in FIG. 3.28 and FIG. 3.31 of “Mixer Topology Selection for a Multi-Standard High Image-Reject Front-End” and “CMOS WIRELESS TRANSCEIVER DESIGN”. In the third structure, a mixer for mixing a complex IF signal and a complex local signal is set as a full-complex mixer for outputting a complex RF signal. The third structure rejects a negative frequency component of the complex RF signal of the mixer output using a polyphase filter. However, even though the structure is theoretically excellent, the polyphase filter is configured by an RC circuit, so loss becomes large and a band becomes narrow. There are additional problems in that loss is further increased, an image rejection ratio of a filter output is reduced, and utility is degraded when the number of steps increases to obtain a high attenuation level or a wide band. These problems are the first problems of the upconverter.
“CMOS WIRELESS TRANSCEIVER DESIGN” proposes a method for obtaining a complex IF signal to be input to the above-described full-complex mixer from a baseband signal in the half-complex mixer, as illustrated in FIG. 3.28 and FIG. 3.31. However, this method has a problem of an increase of power consumption and a problem of spurious reception occurs due to the increased number of local signal oscillators because the number of mixers and the number of local signal oscillators are increased. These problems are the second problems of the upconverter.
In addition to the above-described first and second problems, the low-IF scheme has a problem in that a circuit size increases because a local oscillator 401 for outputting orthogonal local signals of sin and cos and two multipliers, i.e., mixers 402 and 403 are needed as illustrated in FIG. 32. As compared with the heterodyne scheme requiring only a single local signal and a single mixer, the low-IF scheme has a problem in that power consumption increases. These problems are the third problems of the upconverter.
B. Background Technology of Downconverter and Upconverter of Zero-IF Scheme
A downconverter and an upconverter of the zero-IF scheme are provided as configuration examples of an upconverter and a downconverter for converting an RF or IF signal to a complex baseband signal in which a circuit is very simplified and is easily miniaturized.
C. Downconverter of Zero-IF Scheme
FIG. 44 illustrates the downconverter of the zero-IF scheme, and FIG. 56 illustrates the upconverter of the zero-IF scheme.
FIG. 45 illustrates a mixing process in a downconverter 46 of the zero-IF scheme illustrated in FIG. 44. The downconverter 46 multiplies (or half-complex mixes) a real RF signal srf(t) and a complex local signal Lrf(t) of the same frequency as a frequency fc of an associated real RF signal output from a local oscillator 602 in a half-complex mixer 603. Then, the downconverter 46 performs a frequency conversion process in which the center frequency is frequency zero (DC). Then, the downconverter 46 converts an associated signal to a complex baseband signal sbb(t). Then, the downconverter 46 inputs the complex baseband (BB) signal sbb(t) to a demodulator.
In the operation of the half-complex mixer 603 for converting only a positive component of a real RF signal to a baseband signal, a signal Slm(t)Lle(t) based on a negative frequency component of the real RF signal overlaps with a target signal Slp(t)Ll(t) at frequency zero of the complex BB signal sbb(t). The overlap occurs because of a negative component Slm(t) (or a complex conjugate signal of the positive component) of the real RF signal generated from Error Vector Magnitude (EVM) due to imbalance between I and Q components and an error component Lle(t) of the complex local signal. For this reason, it is difficult for only the target signal to be extracted.
A real RF signal, a complex local signal, and a complex baseband signal in a process as illustrated in FIG. 45 are expressed by Equations (1), (2), and (3), respectively.
                                                                                          s                  rf                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                                                                  s                        li                                            ⁡                                              (                        t                        )                                                              +                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                        li                                            ⁡                                              (                        t                        )                                                              -                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                                                                                        =                            ⁢                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                    lm                                    ⁡                                      (                    t                    )                                                                                                          Equation        ⁢                                  ⁢                  (          1          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          2          )                                                              s            bb                    ⁡                      (            t            )                          =                                            (                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                    lm                                    ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                l                            ⁡                              (                t                )                                              +                                    (                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                    lm                                    ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                le                            ⁡                              (                t                )                                                                        Equation        ⁢                                  ⁢                  (          3          )                    
FIG. 57 illustrates a mixing process in an upconverter 66 of the zero-IF scheme illustrated in FIG. 56. The upconverter 66 multiplies (or half-complex mixes) a complex baseband signal sbb(t) and a complex local signal Lrf(t) with a frequency fc of an RF signal srf(t) output from a local oscillator 910 in a half-complex mixer 911. Then, the upconverter 66 performs frequency conversion to the RF signal frequency fc and outputs a real part of the RF signal. Because the EVM-related degradation (in an associated process as in the downconverter), an RF signal Sl(t)Ll(t) overlaps with a signal Sl*(t)Lle*(t) converted to a reverse frequency with respect to the RF signal frequency fc. The overlap occurs due to an error component Lle(t) of a complex local signal and a negative frequency of the RF signal frequency fc in the complex baseband signal sbb(t) when the frequency conversion to the RF signal and the reverse frequency conversion are performed and a real part is output. For this reason, it is difficult for only the target signal to be extracted.
A complex baseband signal, a complex local signal, and a complex RF signal in a process as illustrated in FIG. 57 are expressed by Equations (4), (5), and (6), respectively.
                                          s            bb                    ⁡                      (            t            )                          =                                                            s                li                            ⁡                              (                t                )                                      +                                          js                lq                            ⁡                              (                t                )                                              =                                    s              l                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          4          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          5          )                                                              S            rf                    ⁡                      (            t            )                          =                              1            2                    ⁢                      (                                                                                s                    l                                    ⁡                                      (                    t                    )                                                  ⁢                                                      L                    le                                    ⁡                                      (                    t                    )                                                              +                                                                    s                    l                    *                                    ⁡                                      (                    t                    )                                                  ⁢                                                      L                    l                    *                                    ⁡                                      (                    t                    )                                                              +                                                                    s                    l                    *                                    ⁡                                      (                    t                    )                                                  ⁢                                                      L                    le                    *                                    ⁡                                      (                    t                    )                                                              +                                                                    s                    l                                    ⁡                                      (                    t                    )                                                  ⁢                                                      L                    l                                    ⁡                                      (                    t                    )                                                                        )                                              Equation        ⁢                                  ⁢                  (          6          )                    
The upconverter and downconverter of the zero-IF scheme have an advantage in that they can be miniaturized, as compared with those for performing multi-step frequency conversion in the above-described heterodyne scheme. When real and imaginary axis signals I and Q of a local signal are not orthogonal after processing in the above-described mixer, a problem of the EVM-related degradation due to instability occurs. A problem of a DC offset occurs when leakage of the local signal is self-received in the mixer. When the second-order intermodulation (IM2) occurs due to non-linearity of the mixer, a problem of interference to a target signal occurs.
The upconverter of the zero-IF scheme has an advantage in that it can be miniaturized, as compared with an upconverter for performing the above-described multi-step frequency conversion. However, a problem of the EVM-related degradation and a problem of carrier leakage corresponding to a DC offset of the zero-IF scheme occur. Among these problems, the EVM-related degradation significantly influences to limit communication rate when multi-level modulation is performed according to a high communication rate. The EVM-related degradation needs to be prevented.
When real and imaginary axis signals I and Q of a local signal are not orthogonal after processing in the mixer, the problem of the EVM-related degradation due to instability occurs as described above. To prevent the EVM-related degradation, technology is being developed to improve characteristics of a circuit that reduces amplitude error and phase error between the real and imaginary axis signals I and Q of the local signal and reduces error between transistors configuring the mixer. Many technologies are being developed to prevent the EVM-related degradation by compensating for error between the real and imaginary axis signals I and Q according to a digital signal process after a complex baseband signal is converted to a digital signal.
However, the improvement of circuit characteristics is limited because of instability of an analog circuit. Specifically, degradation due to interference between codes in the multi-level modulation and degradation due to interference between carriers in Orthogonal Frequency Division Multiplexing (OFDM) occur. Due to these degradations, limitations are present in increasing a communication rate of the zero-IF scheme. There are problems in that compensation technology based on digital signal processing increases power consumption for complex processing, and cannot sufficiently prevent the EVM-related degradation. These problems are the first problems of the zero-IF scheme.
Image rejection technology of the image rejection downconverter of “Mixer Topology Selection for a Multi-Standard High Image-Reject Front-End” and “CMOS WIRELESS TRANSCEIVER DESIGN” (FIG. 3.25(b)) with a structure similar to the downconverter of the zero-IF scheme will be described as an example of means for rejecting a negative frequency component of a real RF signal in the downconverter of the above-described zero-IF scheme. When the downconverter with the above-described structure is generalized, the structure of FIG. 46 results. A downconverter 47 illustrated in FIG. 46 uses a complex-coefficient filter 513 in which positive and negative frequency characteristics are different before a real RF signal is frequency-converted. The downconverter 47 can obtain a complex RF signal srf(t) in which a negative frequency component of a real RF signal is rejected. A full-complex mixer 610 mixes the complex RF signal srf(t) and a complex local signal Lrf(t) and performs down-conversion to a complex IF signal sif(t).
FIG. 47 illustrates a process when the complex-coefficient filter 513 is absent in the downconverter 47 of FIG. 46. When the complex-coefficient filter 513 is absent, a negative frequency component Lle(t) occurs due to the complex local signal Lrf(t) and instability of the full-complex mixer 610. An associated negative frequency component is a factor for generating an image frequency signal (Slm(t)Lle(t)+S(t)Lle(t)).
A real RF signal, a complex local signal, and a complex IF signal in a process as illustrated in FIG. 47 are expressed by Equations (7), (8), and (9), respectively.
                                                                                          s                  rf                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                                                                  s                        li                                            ⁡                                              (                        t                        )                                                              +                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                        li                                            ⁡                                              (                        t                        )                                                              -                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                      ⁢                                                                                                                  s                                                  2                          ⁢                          i                                                                    ⁡                                              (                        t                        )                                                              +                                                                  js                                                  2                          ⁢                          q                                                                    ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                                                  2                          ⁢                          i                                                                    ⁡                                              (                        t                        )                                                              -                                                                  js                                                  2                          ⁢                          q                                                                    ⁡                                              (                        t                        )                                                                              2                                                                                                        =                            ⁢                                                (                                                                                    s                        lp                                            ⁡                                              (                        t                        )                                                              +                                                                  s                                                  2                          ⁢                          p                                                                    ⁡                                              (                        t                        )                                                                              )                                +                                  (                                                                                    s                        lm                                            ⁡                                              (                        t                        )                                                              +                                                                  s                                                  2                          ⁢                          m                                                                    ⁡                                              (                        t                        )                                                                              )                                                                                        Equation        ⁢                                  ⁢                  (          7          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          8          )                                                              s            if                    ⁡                      (            t            )                          =                                            (                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                                          2                      ⁢                      p                                                        ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                l                            ⁡                              (                t                )                                              +                                    (                                                                    s                    lm                                    ⁡                                      (                    t                    )                                                  +                                                      s                                          2                      ⁢                      m                                                        ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                le                            ⁡                              (                t                )                                                                        Equation        ⁢                                  ⁢                  (          9          )                    
When the complex-coefficient filter 513 rejects a negative frequency component, interference due to an image frequency signal can be reduced. FIG. 48 illustrates a rejection process. Only a component of a complex RF signal srf′(t) in which the negative frequency component is rejected is frequency-converted to the complex IF signal sif(t). Because an amount of attenuation of the complex-coefficient filter as illustrated in FIG. 48 is finite, the negative frequency component cannot be completely rejected. In addition to an image rejection ratio of the full-complex mixer 610, interference due to a signal of a negative frequency component of the complex RF signal is reduced by an amount of attenuation of a negative frequency of the complex-coefficient filter 513.
A real RF signal, a complex RF signal, a complex local signal, and a complex IF signal in a process as illustrated in FIG. 48 are expressed by Equations (10), (11), (12), and (13), respectively.
                                                                                          s                  rf                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                                                                  s                        li                                            ⁡                                              (                        t                        )                                                              +                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                        li                                            ⁡                                              (                        t                        )                                                              -                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                      ⁢                                                                                                                  s                                                  2                          ⁢                          i                                                                    ⁡                                              (                        t                        )                                                              +                                                                  js                                                  2                          ⁢                          q                                                                    ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                                                  2                          ⁢                          i                                                                    ⁡                                              (                        t                        )                                                              -                                                                  js                                                  2                          ⁢                          q                                                                    ⁡                                              (                        t                        )                                                                              2                                                                                                        =                            ⁢                                                (                                                                                    s                        lp                                            ⁡                                              (                        t                        )                                                              +                                                                  s                                                  2                          ⁢                          p                                                                    ⁡                                              (                        t                        )                                                                              )                                +                                  (                                                                                    s                        lm                                            ⁡                                              (                        t                        )                                                              +                                                                  s                                                  2                          ⁢                          m                                                                    ⁡                                              (                        t                        )                                                                              )                                                                                        Equation        ⁢                                  ⁢                  (          10          )                                                              s            rf            ′                    ⁡                      (            t            )                          =                              s            lp                    ⁡                      (            t            )                                              Equation        ⁢                                  ⁢                  (          11          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          12          )                                                              s            if                    ⁡                      (            t            )                          =                                            (                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                                          2                      ⁢                      p                                                        ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                l                            ⁡                              (                t                )                                              +                                    (                                                                    s                    lm                                    ⁡                                      (                    t                    )                                                  +                                                      s                                          2                      ⁢                      m                                                        ⁡                                      (                    t                    )                                                              )                        ⁢                                          L                le                            ⁡                              (                t                )                                                                        Equation        ⁢                                  ⁢                  (          13          )                    
When the downconverter 47 illustrated in FIG. 46 is applied to the zero-IF scheme, a mixing process rejects a negative frequency component of the real RF signal as illustrated in FIG. 49, such that only a target signal can be obtained at frequency zero.
A real RF signal, a complex RF signal, a complex local signal, and a complex baseband signal in a process as illustrated in FIG. 49 are expressed by Equations (14), (15), (16), and (17), respectively.
                                                                                          s                  rf                                ⁡                                  (                  t                  )                                            =                            ⁢                                                                                                                  s                        li                                            ⁡                                              (                        t                        )                                                              +                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                +                                                                                                    s                        li                                            ⁡                                              (                        t                        )                                                              -                                                                  js                        lq                                            ⁡                                              (                        t                        )                                                                              2                                                                                                        =                            ⁢                                                                    s                    lp                                    ⁡                                      (                    t                    )                                                  +                                                      s                    lm                                    ⁡                                      (                    t                    )                                                                                                          Equation        ⁢                                  ⁢                  (          14          )                                                              s            rf            ′                    ⁡                      (            t            )                          =                              s            lp                    ⁡                      (            t            )                                              Equation        ⁢                                  ⁢                  (          15          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          16          )                                                              s            bb                    ⁡                      (            t            )                          =                                                            s                lp                            ⁡                              (                t                )                                      ⁢                                          L                l                            ⁡                              (                t                )                                              +                                                    s                lp                            ⁡                              (                t                )                                      ⁢                                          L                le                            ⁡                              (                t                )                                                                        Equation        ⁢                                  ⁢                  (          17          )                    D. Upconverter of Zero-IF Scheme
Next, an upconverter of the zero-IF scheme will be described. Like the downconverter of the zero-IF scheme, the upconverter of the zero-IF scheme uses a full-complex mixer and a complex-coefficient filter and can reject a distortion error of a transmission signal due to the EVM-related degradation resulting from imbalance between I and Q.
FIG. 58 illustrates a structure of an upconverter 67 with a full-complex mixer 920 and a complex-coefficient filter 710. A mixing process of the upconverter 67 is illustrated in FIG. 59. The full-complex mixer 920 mixes a complex baseband signal sbb(t) and a complex local signal Lrf(t) and then outputs a complex RF signal srf(t). Then, the upconverter 67 uses a complex-coefficient filter 710 in which positive and negative frequency characteristics are different in a complex RF signal. After a negative frequency component of the complex RF signal is rejected, a real part is extracted. A real RF signal (½(Sl(t)Ll(t)+Sl*(t)Ll*(t))) can be obtained as a target.
A complex baseband signal, a complex local signal, a complex RF signal, and an RF signal in a process as illustrated in FIG. 59 are expressed by Equations (18), (19), (20), and (21), respectively.
                                          s            bb                    ⁡                      (            t            )                          =                                                            s                li                            ⁡                              (                t                )                                      +                                          js                lq                            ⁡                              (                t                )                                              =                                    s              l                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          18          )                                                              L            rf                    ⁡                      (            t            )                          =                                            L              l                        ⁡                          (              t              )                                +                                    L              le                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          19          )                                                              s            rf                    ⁡                      (            t            )                          =                                            s              l                        ⁡                          (              t              )                                +                                    L              rf                        ⁡                          (              t              )                                                          Equation        ⁢                                  ⁢                  (          20          )                                                              s                                                                      ⁢              rf                                ⁢                                          ⁢                      (            t            )                          =                              Re            ⁡                          [                                                                    s                                                                                                              ⁢                      l                                                        ⁢                                                                          ⁢                                      (                    t                    )                                                  ⁢                                                                  +                                                                  ⁢                                                      L                                                                                                              ⁢                      l                                                        ⁢                                                                          ⁢                                      (                    t                    )                                                              ]                                =                                    1                                                                                ⁢                2                                      ⁢                                                  ⁢                          (                                                                    s                                                                                                              ⁢                      l                                                        ⁢                                                                          ⁢                                      (                    t                    )                                    ⁢                                                                          ⁢                                      L                                                                                                              ⁢                      l                                                        ⁢                                                                          ⁢                                      (                    t                    )                                                  ⁢                                                                  +                                                                  ⁢                                                      s                                                                                                              ⁢                      l                                        *                                    ⁢                                                                          ⁢                                      (                    t                    )                                    ⁢                                                                          ⁢                                      L                                                                                                              ⁢                      l                                        *                                    ⁢                                                                          ⁢                                      (                    t                    )                                                              )                                                          Equation        ⁢                                  ⁢                  (          21          )                    
As compared with the heterodyne scheme requiring only a single local signal and a single mixer, the conventional low-IF scheme requires a local oscillator 602 or 910 for outputting orthogonal local signals of sin and cos and two multipliers, i.e., mixers 604 and 605 or mixers 912 and 913, are required as illustrated in the structure of FIG. 44 or 56. In the structure of FIG. 46, FIG. 58, or a structure of FIG. 3.28 and FIG. 3.31 of “CMOS WIRELESS TRANSCEIVER DESIGN”, the number of complex-coefficient filters or the number of mixers increases and therefore a circuit size increases. For this reason, the power consumption of the zero-IF scheme is greater than that of the heterodyne scheme. This shortcoming is the second problem of the zero-IF scheme.
As illustrated in FIG. 3.28 and FIG. 3.31, “CMOS WIRELESS TRANSCEIVER DESIGN” proposes a method for generating a complex signal using a half-complex mixer. However, power consumption increases because the number of mixers and the number of local signal oscillators are increased and spurious reception occurs due to the increased number of local signal oscillators. These shortcomings are the third problems of the zero-IF scheme.
As described above, the downconverter of the low-IF scheme has the first to fifth problems, the upconverter of the low-IF scheme has the first to third problems, and the downconverter and upconverter of the zero-IF scheme have the first to third problems.
The important problems in the downconverter and upconverter of the low-IF scheme occur when a sufficient image rejection ratio cannot be obtained and power consumption increases.
The important problems in the downconverter and upconverter of the zero-IF scheme occur when EVM-related degradation occurs and power consumption increases.
There are increasing market needs for the downconverter and upconverter of the low-IF scheme and the zero-IF scheme capable of processing a broadband or multi-band RF signal. The problems of the low-IF scheme and the zero-IF scheme must be able to be addressed and the broadband or multi-band RF signal must be able to be processed.